1. Field of the Invention
The present invention relates to a semiconductor memory, and more particularly, it relates to a semiconductor memory using resistive memory elements.
2. Description of the Related Art
In recent years, many types of memory storing information based on this new fundamental principle have been suggested. One of these memories is a spin transfer type magnetoresistive random access memory (e.g., in “A Novel Nonvolatile Memory with Spin Torque Transfer Magnetization Switching: Spin-RAM”, IEDM2005 Technical Digest pp. 473-476).
In a magnetoresistive random access memory (MRAM), a magnetic tunnel junction (MTJ) element is used as a memory element. A MTJ element has a structure in which two magnetic layers (ferromagnetic layers) sandwich an insulating layer (a tunnel barrier) therebetween and utilizes a magnetic tunnel effect in which a resistance value varies according to a relative magnetizing direction (a spin direction) of the two magnetic layers to discriminate data “1” and data “0”.
Usually, an antiferromagnetic layer is arranged on one side of the two magnetic layers. A magnetizing direction of one ferromagnetic layer is fixed by the antiferromagnetic layer, and a magnetizing direction of the ferromagnetic layer on the other side alone is variable. The ferromagnetic layer having the magnetizing direction fixed in this manner is called a magnetization invariable layer, whereas the ferromagnetic layer having the variable magnetizing direction is called a magnetization free layer. In the MRAM, changing a magnetizing direction of a magnetization free layer enables rewriting data in the MTJ element.
A plurality of memory cells are provided in a memory cell array of the MRAM. To one memory cell are connected one word line and two bit lines forming one pair of bit lines.
One memory cell has a so-called a 1Tr+1MTJ structure formed of, e.g., one MTJ element and an MIS (Metal-Insulator-Semiconductor) transistor that functions as a selective switch element with respect to the MTJ element.
In the spin transfer torque type MRAM, a current amount (a reversing threshold current) required for reversal of spin transfer magnetization of the MTJ element is defined by a current density flowing through the MTJ element. Therefore, with scaling of the MTJ element, the reversing threshold current can be also scaled. That is, a cell size of each memory cell can be relatively easily reduced in order to increase a storage capacity of the MRAM.
However, along with advances in miniaturization of memory cells, cutoff resistance properties of the MIS transistor constituting the memory cells cannot be sufficiently assured, and a leak current (which will be referred to as a cutoff current hereinafter) is produced.
The memory cell array in the MRAM has a structure in which a plurality of memory cells are connected with one pair of bit lines in common. Further, the MRAM selects one memory cell from the plurality of memory cells connected with the same pair of bit lines by selecting a word line, and executes a read or write operation with respect to the selected memory cell.
In the read operation of the MRAM, a potential of one bit line in the pair of bit lines is set to a high potential, and a potential of the other bit line is set to a low potential. As a result, a read current flows through the MTJ element in the selected cell, and a potential of the bit line fluctuates in accordance with a resistance value of this MTJ element. Detecting a fluctuation amount of this potential enables discriminating data.
As explained above, when the cutoff resistance properties of the MIS transistor in the memory cell are not assured and a cutoff current is produced from a non-selected cell, the potential of the bit line fluctuates due to this cutoff current, and a read margin of the MRAM is degraded.